High efficiency audio reproduction

ABSTRACT

A device is arranged for producing a driving signal (V M ) for a transducer ( 7 ), such as a loudspeaker. The driving signal has a frequency substantially equal to a resonance frequency of the transducer and an amplitude controlled by an external signal (V E ). The device is arranged for automatically adjusting the frequency of the driving signal to the resonance frequency of the transducer, using a control path ( 8 ). The device may be part of a frequency adaptation device ( 1 ) for adapting a frequency range of an audio signal to the transducer ( 7 ).

The present invention relates to high efficiency audio reproduction.More in particular, the present invention relates to a device capable ofproducing a high efficiency driving signal for a transducer and acorresponding method therefore. The present invention further relates toa device for adapting the frequency range of an audio signal to atransducer and a corresponding method therefore.

It is well known that audio frequencies range from approximately 20 Hzto approximately 20 kHz. While the middle range (approx. 1-10 kHz) canbe reliably reproduced by regular loudspeakers, special transducers aretypically required for the lower and higher frequency ranges. Highfidelity audio systems typically include small transducers (tweeters)for reproducing the high audio frequency range, and relatively largetransducers (woofers) for the low range. The transducers required tofaithfully reproduce the lowest audible frequencies (approx. 20-100 Hz)at a suitable sound volume take up a substantial amount of space.However, there is an increasing demand for miniature audio sets. It isobvious that the requirements of large transducers and small audioequipment are incompatible.

It has been proposed to solve this problem by using psycho-acousticphenomena such as “virtual pitch”. By creating harmonics oflow-frequency signal components it is possible to suggest the presenceof such signal components without actually reproducing them.

United States Patent Specification U.S. Pat. No. 6,134,330 (Philips),for example, discloses an audio system provided with enhancing means forenhancing the audio signal. These known enhancing means comprise aharmonics generator for generating harmonics of a first part of theaudio signal so as to create the illusion that the perceived audiosignal includes lower frequency components than are really available.

Although this known solution works remarkably well, it is no substitutefor actually reproducing low-frequency (bass) signal components.

It is therefore an object of the present invention to overcome these andother problems of the Prior Art and to provide a device for and a methodof reproducing audio signals which allows a more efficient reproductionof the entire audio frequency range, and in particular of low-frequencysignal components.

It is a further object of the present invention to provide a device anda method for reproducing audio signals which audio signals can beautomatically adjusted to the transducer so as to provide maximumefficiency.

Accordingly, the present invention provides a device for producing adriving signal for a transducer, the driving signal having a frequencysubstantially equal to a resonance frequency of the transducer and anamplitude controlled by an external signal, which device is providedwith control means for automatically adjusting the frequency of thedriving signal to the resonance frequency of the transducer.

By driving the transducer at a frequency that is substantially equal toits resonance frequency, the transducer is extremely efficient,producing the maximum sound output power at a given electrical inputpower. By providing control means for automatically adjusting thefrequency of the driving signal to the resonance frequency of thetransducer, it is ensured that the transducer is always operating at itsmaximum efficiency, irrespective of the temperature, atmosphericpressure, and other factors. In addition, any spread in the transducercharacteristics is automatically compensated. It is therefore possibleto replace the transducer with a similar, but not identical, othertransducer, without the need for manual tuning of the device.

The control means may control and/or adjust the driving signal frequencyon the basis of one or more transducer properties, such as the(instantaneous) impedance. It is known that the impedance of thetransducer is frequency-dependant. In particular, the impedance of atransducer involves a phase shift that is equal to zero at the(dominant) resonance frequency of the transducer. The present inventorhas realized that determining this phase shift and controlling thefrequency of the driving signal so as to minimize the phase shift is aconvenient way of matching the driving signal frequency with theresonance frequency. Accordingly, in a preferred embodiment of thedevice of the present invention the control means comprise phasedetermining means for determining any phase shift introduced by thetransducer.

More in particular, the phase determining means preferably comprise acombination unit for combining a first signal representative of thephase of the driving signal's voltage and a second signal representativeof the driving signal's current so as to produce a phase differencesignal, and a control unit for producing a frequency control signal onthe basis of the phase difference signal. Such an arrangement allows thephase of the driving signal to be used to control the frequency of thesame signal.

In a first embodiment, the first signal is the driving signal. That is,the driving signal is combined with a second signal to produce a phasedifference signal. In a second embodiment, the first signal is anauxiliary oscillator signal. In this second embodiment, an auxiliaryoscillator signal is provided, typically but not necessarilyphase-shifted over 90° (π/2 radians), in addition to a main oscillatorsignal from which the driving signal is derived. With such an auxiliaryoscillator signal an improved phase detection may be achieved.

In an advantageous embodiment, the device further comprises a phasecompensation unit for introducing a compensatory phase shift in theauxiliary oscillator signal so as to produce a phase shifted auxiliaryoscillator signal. The compensatory phase shift is designed to besubstantially equal to any phase shift introduced by amplifiers, filtersand any other components.

The device may further comprise a resistor arranged in series with thetransducer for producing the second signal in response to the drivingcurrent. That is, the driving current passing through the resistorproduces the second signal. As a result, the phase of the second signalis equal to the phase of the driving current Additionally, oralternatively, the device of the present invention may further comprisean acceleration detector for detecting an acceleration of thetransducer, and/or a displacement detector for detecting a displacementof the transducer.

The device of the present invention may further comprise a generator forgenerating an oscillation signal having a frequency substantially equalto the resonance frequency of the transducer, and a further combinationunit for combining the oscillation signal with an amplitude controlsignal so as to produce the amplitude controlled driving signal.

In addition, the device may further comprise an amplifier for amplifyingthe driving signal, and/or a low-pass filter for filtering the drivingsignal.

The present invention further provides a frequency adaptation device foradapting a frequency range of an audio signal to a transducer, thedevice comprising:

-   detection means for detecting first signal components in a first    audio frequency range,-   generator means for generating second signal components in a second    audio frequency range,-   amplitude control means for controlling the amplitude of the second    signal components in response to the amplitude of the first signal    components, and-   control means for determining the second audio frequency range on    the basis of transducer properties,-   wherein the second audio frequency range is substantially narrower    than the first audio frequency range, and wherein the transducer has    a maximum efficiency at the second audio frequency range.

By generating second signal components in a second audio frequency rangewhich is substantially narrower than the first frequency range, theamplitude of the second signal components being controlled in responseto the amplitude of the first signal components, the energy of the audiosignal is concentrated in the second frequency range. As a result, thebandwidth of the first frequency range is effectively reduced and theenergy of the audio signal is concentrated in a substantially narrower(second) range. This has the advantage that the energy of the audiosignal can be concentrated in a range in which a transducer isparticularly efficient, thus resulting in a more efficient soundproduction. The second frequency range can be extremely narrow,effectively comprising only the frequency at which the transducer ismost efficient, typically the resonance frequency. It is preferred thatthe second frequency range is partially or entirely within the firstfrequency range.

In a preferred embodiment, the control means are arranged forautomatically controlling the second frequency range on the basis oftransducer properties, such as the (instantaneous) impedance.

The present invention also provides a method of producing a drivingsignal for a transducer, the driving signal having a frequencysubstantially equal to a resonance frequency of the transducer and anamplitude controlled by an external signal, the method comprising thestep of automatically adjusting the frequency of the driving signal tothe resonance frequency of the transducer.

The method may advantageously further comprise the step of determiningany phase shift introduced by the transducer. In a preferred embodiment,the method of the present invention further comprises the steps ofcombining a first signal representative of the phase of the drivingsignal's voltage and a second signal representative of the drivingsignal's current so as to produce a phase difference signal, andproducing a frequency control signal on the basis of the phasedifference signal. The first signal may be the driving signal or anauxiliary oscillator signal. In the latter case, the method may furthercomprise the step of introducing a compensatory phase shift in theauxiliary oscillator signal so as to produce a phase shifted auxiliaryoscillator signal. The second signal may be produced in response to thedriving current of the transducer.

In further advantageous embodiments, the method may additionally oralternatively comprise the step of detecting an acceleration or adisplacement of the transducer.

It is preferred that the method further comprises the steps ofgenerating an oscillation signal having a frequency substantially equalto the resonance frequency of the transducer, and combining theoscillation signal with an amplitude control signal so as to produce theamplitude controlled driving signal.

The method may further comprise the steps of amplifying and/or filteringthe driving signal.

The present invention additionally provides a frequency adaptationmethod for adapting a frequency range of an audio signal to atransducer, the method comprising the steps of selecting a frequencyrange, detecting signals in the selected frequency range, and producinga driving signal for a transducer in accordance with the method definedabove.

The present invention will further be explained below with reference toexemplary embodiments illustrated in the accompanying drawings, inwhich:

FIG. 1 schematically shows a first embodiment of a device according tothe present invention.

FIG. 2 schematically shows a second embodiment of a device according tothe present invention.

FIG. 3 schematically shows a third embodiment of a device according tothe present invention.

FIG. 4 schematically shows a fourth embodiment of a device according tothe present invention.

FIG. 5 schematically shows an audio system in accordance with thepresent invention.

FIG. 6 schematically shows a first and a second frequency range inaccordance with the present invention.

The device 1 shown merely by way of non-limiting example in FIG. 1comprises a first filter 2, a detector 3, a second filter 4, acombination unit 5, a generator 6, and a control path 8. A transducer 7is coupled to the combination unit 5.

The device 1 of FIG. 1, which serves to adapt a frequency range of anaudio signal to a transducer, comprises two parts: a first partconsisting of the first filter 2, the detector 3 and the (optional)second filter 4, and a second part consisting of the combination unit 5,the generator 6 and the control path 8. The first part serves to producean amplitude control signal on the basis of a selected frequency rangeof an (audio) input signal, while the second part serves to produce anamplitude controlled transducer driving signal.

The band-pass (first) filter 2, the detector 3 and the low-pass (second)filter 4 produce an amplitude control (that is, modulating) signal whichis based on an input signal V_(in). This input signal V_(in) istypically an audio signal, in particular the bass (low-frequency) partof an audio signal. The band-pass filter 2, which in some embodimentsmay be replaced with a low-pass filter, selects a frequency range andoutputs an audio signal V_(F) having a limited frequency-range, forexample 20 Hz to 120 Hz. The signal components of this selectedfrequency range are detected in the detector 3, which produces anenvelope (that is, amplitude control) signal V_(E). The detector 3preferably is an envelope detector known per se but which may also be apeak detector known per se. In a very economical embodiment, thedetector 3 may be constituted by a diode. It is noted that the second(low-pass) filter 4 merely serves to smooth the envelope signal V_(E)and may be omitted.

As explained above, the output signal V_(E) of the (envelope) detector 3represents the amplitude of the input signal components present in afirst frequency range (I in FIG. 6) selected by the filter 2. Thissignal V_(E) is subsequently used as an amplitude control signal. Tothis end, the combination unit 5, which in the embodiment shown isconstituted by a multiplier, combines (multiplies) this amplitudecontrol signal V_(E) with an oscillator signal V₀ generated by thegenerator (oscillator) 6 so as to form a driver signal V_(M) for drivingthe transducer 7. This driver signal V_(M) will have a frequency definedby the generator 6 and an amplitude defined by the signal V_(E)′ (or, ifthe second filter 4 is not present, V_(E)).

As will later be explained in more detail with reference to FIG. 6, thefrequency of the generator 6 is substantially equal to the resonancefrequency of the transducer 7. This allows the transducer to operate atits maximum efficiency. A suitable transducer is described in EuropeanPatent Application No. 03103396.2 (PHNL031135). Although the transducer7 is typically constituted by a loudspeaker, other transducers can alsobe envisaged, such as so-called “shakers” that cause other objects tovibrate. The single transducer 7 may be replaced with a group of two ormore transducers.

The control path 8 serves to control the frequency of the generator 6and, more in particular, to keep the generator frequency substantiallyat a selected resonance frequency of the transducer (transducerstypically have multiple resonance frequencies but preferably theresonance frequency is selected at which the desired sound output isachieved), for example 60 Hz. The control path 8 allows the generator 6to adjust the frequency (and preferably also the phase) in dependence ontransducer parameters such as the (instantaneous) impedance (or itsabsolute value), the actual movement of the vibration surface of thetransducer, and/or sound pressure.

It will be clear to those skilled in the art that these parameters makeit possible to determine the efficiency (the output power divided by theinput power) of the transducer. As the efficiency will typically varywith the frequency, an adjustment of the frequency will allow theefficiency to be optimized. To this end the generator may introducesmall (and possibly random) frequency variations to determine theefficiency at various frequencies around the current value. If at any ofthose alternative frequencies the efficiency is greater, the set valueof the frequency may be altered. In this way, automatic tuning of thegenerator 6 may be provided, even in the absence of the control path 8.However, it is preferred to directly control the frequency of thegenerator without introducing any frequency variations.

To directly control the frequency of the generator 6, the control path 8may feed a suitable frequency control signal to the generator 6, thisfrequency control signal being derived from one or more transducerparameters. In a preferred embodiment, the phase of the current ILpassing through the transducer 7 is used to control the generatorfrequency, as schematically shown in FIG. 2. The generator 6 ispreferable constituted by a VCO (Voltage Controlled Oscillator) knownper se.

The device 1 of FIG. 2 also comprises a (first) filter 2, an envelopedetector 3, a combination unit (multiplier) 5, and a generator 6. Thesecond filter 4 has been deleted, while an amplifier 9 has been insertedbetween the combination unit 5 and the transducer 7 to provide asuitable driving current I_(L) for the transducer 7. The amplifier 9,and consequently the driving current I_(L), is controlled by the drivingvoltage V_(M).

In the exemplary embodiment of FIG. 2, the control path 8 is shown tocomprise a resistor 10, a further (or second) combination unit 11, and acontrol unit 12. The driving current I_(L) passes through the transducer7 and the resistor 10 to ground (or a suitable return connection),generating a resistor voltage V_(R) over the resistor 10. Thecombination unit 11, which may also be constituted by a multiplier,combines (multiplies) this resistor voltage V_(R) with the drivingvoltage V_(M) to produce a combined voltage V_(D) that is passed to thecontrol unit 12. The control unit 12, which may be constituted by alow-pass filter, converts the combined voltage V_(D) into a suitablegenerator control voltage V_(C) that controls the frequency of thegenerator 6.

As mentioned above, the control path determines the phase of the drivingcurrent I_(L). In mathematical terms, this may be expressed as follows.

The driving voltage V_(M) is the product of the generator signal V₀ andthe amplitude signal V_(E):V _(M) =V _(E) ·V ₀ =V _(E)·sin(ωt),where ω=2π. f, and f being the generator frequency. The magnitude of theresistor voltage V_(R) over the resistor R is C times the magnitude ofthe driving voltage V_(M), where C depends on the impedance of thetransducer 7, the resistor 10, and the gain of the amplifier 9. As thetransducer 7 introduces a phase shift φ, the resistor voltage V_(R) cannow be written as:V _(R) =C˜V _(E)·sin(ωt+φ)

The phase shift φ is frequency-dependent and substantially equals zeroat the resonance frequency of the transducer 7. When this resistorsignal V_(R) is multiplied with the driving signal V_(M) by thecombination unit 11, the combined signal V_(D) can be written as:V _(D) =V _(M) ·V _(R) =V _(E)·sin(ωt)·C·V _(E)·sin(ωt+φ), orV _(D)=½·C·V _(E) ²·{cos(φ)−cos(2ωt+φ}.Low-pass filtering at the control unit 12 (transfer function H) willresult in:V _(C)=½·C·V _(E) ²·cos(φ),which is independent of the frequency ω (=2π·f) and is at a maximum forφ=0. The generator 6 is therefore, in this embodiment, arranged formaximizing the control voltage V_(C), as this will make the generatorfrequency equal to the resonance frequency.

It is noted that the circuit arrangement of FIG. 2 is merely exemplaryand that the principles of the present invention can be applied equallywell in other circuit arrangements. For example, the resistor 10 couldbe arranged between the amplifier 9 and the transducer 7. Alternatively,an additional transducer (pick-up element) could be used to determine(the phase of) the current passing through the transducer. Also,additional transducers could be used to register the acceleration,velocity and/or excitation of the transducer to determine transducerparameters, such any phase difference introduced by the transducer 7. Ifthe transducer 7 is constituted by a loudspeaker, for example, anacceleration detector mounted on the cone can be used, or a displacementdetector, for example one using laser technology.

It is further noted that the device 1 may be implemented using analogand/or digital techniques. In case digital techniques are used, thoseskilled in the art will recognize that suitable D/A (digital/analog) andA/D (analog/digital) converters may be present in the device 1.In-digital embodiments the control unit 12 may be constituted by amicrocontroller or a microprocessor.

The embodiment of FIG. 3 also comprises a filter 2, a detector 3, afirst combination unit 5, an amplifier 9, a resistor 10, a secondcombination unit 11, a control unit 12 and a generator 6. However, inthis embodiment the generator 6 is a so-called quadrature generator,arranged for generating two output signals having the same frequency buta mutual phase difference of 90° (=π/2 radians). These generator signalscan therefore be written as sin(ωt) and cos(ωt) respectively. In theembodiment of FIG. 3 this second generator signal V₀′=cos(ωt) is fed tothe further combination unit (multiplier) 11, instead of the drivingsignal V_(M). The output signal V_(D) of the second combination unit 11can now be written as:V _(D) =V ₀ ′·V _(R)=cos(ωt)·C·V _(E)·sin(ωt+φ), orV _(D)=½·C·V _(E)·{sin(2ωt+φ)+sin(φ)}Low-pass filtering by the control unit 12 yields for the control voltageV_(C):V _(C)=½·C·V _(E)·sin(φ)which is independent of the frequency ω (=2π·f) and is equal to zero forφ=0. The generator 6 is therefore, in this embodiment, arranged formaking the control voltage V_(C) equal to zero, as this will make thegenerator frequency equal to the resonance frequency.

It is noted that quadrature oscillators are well known in the art. Aparticularly economical and suitable embodiment of a digital quadratureoscillator comprises a multi-vibrator producing a signal having fourtimes the desired generator frequency, and a flip-flop dividing thesignal by a factor of two. Dividing the resulting signal, which hastwice the desired frequency, by two on the rising edges of the digitalsignal produces the first generator signal V₀=sin(ωt), while dividing onthe falling edges produces the second generator signal V₀′=cos(ωt).Although such an embodiment is advantageous as there is no need for themulti-vibrator signal to be symmetrical, the particular configuration ofthe generator 6 is not essential to the present invention.

The embodiment of FIG. 4 is largely identical to the embodiment of FIG.3. However, a (second) filter 4 has been added to low-pass filter thedetector output signal V_(E), as in FIG. 1. Also, a (third) low-passfilter 13 has been inserted between the combination unit 5 and theamplifier 9 so as to low-pass filter the combined signal V_(M) output bythe combination unit prior to amplification. These optional filters 4and 13 remove any undesirable signal components.

In addition, a phase compensation unit 14 has been added to compensatefor any phase shifts introduced by the amplifier 9 (and/or the thirdfilter 13). This phase compensation unit 14 adds a phase shift Δφ to thesecond generator signal V₀′ so as to obtain a phase shifted secondgenerator signal V₀″=cos(ωt+Δφ). The correct value of the phase shift Δφmay be determined experimentally, temporarily replacing the transducer 7with a resistor to eliminate any transducer phase shift. The addition ofthe phase compensation unit 14 allows a more accurate tuning of thegenerator 6.

In all embodiments, a limiter known per se could be arranged between thecombination unit (multiplier) 11 and the connection between transducer 7and the resistor 10. This would allow the combination unit 11 to beimplemented very economically as an EXOR gate.

An audio system embodying the present invention is schematicallyillustrated in FIG. 5. The audio system 20 is shown to comprise a firstaudio processing unit 21 and a second audio processing unit 1. The firstaudio processing unit 21 receives an audio input signal V_(aud) from asuitable source, such as a CD player, a DVD player, an MPEG player, aradio tuner, a television tuner, a computer hard disc, the Internet, oranother source. The low-frequency part of the audio input signal V_(aud)is passed on to the second audio processing unit 2 as an input signalV_(in), while the mid- and high-frequency parts are processed in thefirst audio processing device 21 and are then fed to the transducer (orset of transducers) 22 via a connection 24. The second audio processingunit 1, which may be identical to the device according to any of FIGS. 1to 4, processes the input signal V_(in) and outputs the processed signalto the transducer 7 via a connection 23. In accordance with the presentinvention, a control path 8 is provided from the transducer 7 to thesecond audio processing unit 1 to adjust the generator frequency of theaudio processing unit 1.

In FIG. 6 a graph showing an audio frequency distribution isschematically depicted. The graph 30 indicates the amplitude Amp(vertical axis) of an audio signal at a particular frequency f(horizontal axis). As shown, the audio signal contains virtually nosignal components below approximately 10 Hz. As the following discussionwill focus on the low-frequency part of the graph 30, the mid- andhigh-frequency parts of the graph have been omitted for the sake ofclarity of the illustration.

In the frequency adaptation device of the present invention, a firstfrequency range is mapped onto a second, smaller frequency range whichis preferably contained in the first frequency range. In thenon-limiting example of FIG. 6, a first frequency range I is the rangefrom 20 Hz to 120 Hz, while a second range II is the range around 60 Hz,for example 55-65 Hz. This first range I substantially covers the“low-frequency” part of an audio signal, whereas the second range II ofFIG. 6 is chosen so as to correspond with a particular transducer, suchas a loudspeaker, and will depend on the characteristics of thetransducer. This second range II corresponds with the frequencies atwhich the transducer is most efficient, resulting in the highest soundproduction.

It will be understood that the size (bandwidth) of the second range IImay also depend on the characteristics of the transducer(s). Atransducer or array of transducers having a wider range of frequenciesat which it is most efficient (possibly multiple resonance frequencies)will benefit from a wider second range II. Transducers or arrays oftransducers having a single most efficient frequency (typically theresonance frequency) may benefit from an extremely narrow second rangeII as this will concentrate all energy in said single frequency.

It is noted that in the example shown the second range II is locatedwithin the first range I. This means that the first range I iseffectively compressed and that no frequencies outside the first rangeare affected.

Accordingly, the device of the present invention can also be defined asa device for driving a transducer with an amplitude modulated signal,the device comprising: generating means for generating a signal having afrequency, modulating means for amplitude modulating the generatedsignal with a modulating signal, feedback means for providing a feedbacksignal from the transducer to the generating means, wherein the feedbackmeans are arranged for adjusting the frequency of the generated signalsuch that it is substantially equal to a resonance frequency of thetransducer.

The present invention may advantageously be applied in electronicconsumer apparatus, such as television sets, audio sets, in-house cinemasystems, car audio systems, laptop computers, and desktop computers. Inparticular in so-called flat panel television sets the present inventionmay improve the sound quality, as in such sets the space available forloudspeakers is typically limited. Replacing a bass speaker with arelatively small resonant transducer, driven at its resonance frequencyin accordance with the present invention, will significantly improve theperception of the bass sound while requiring a very limited amount ofspace.

The present invention is based upon the insight that the driving signalfrequency of a resonant transducer may be accurately tuned by providinga feedback path from the transducer to the generator producing thedriving signal frequency. The present invention benefits from thefurther insight that the phase of the driving current can be usedeffectively to determine whether the transducer is operating at itsresonance frequency. It is noted that any terms used in this documentshould not be construed so as to limit the scope of the presentinvention. In particular, the words “comprise(s)” and “comprising” arenot meant to exclude any elements not specifically stated. Single(circuit) elements may be substituted with multiple (circuit) elementsor with their equivalents.

It will be understood by those skilled in the art that the presentinvention is not limited to the embodiments illustrated above and thatmany modifications and additions may be made without departing from thescope of the invention as defined in the appending claims.

1. A device for producing a driving signal (V_(M)) for a transducer (7),the driving signal having a frequency substantially equal to a resonancefrequency of the transducer and an amplitude controlled by an externalsignal (V_(E)), which device is provided with control means (8; 10, 11,12) for automatically adjusting the frequency of the driving signal tothe resonance frequency of the transducer.
 2. The device according toclaim 1, wherein the control means (8; 10, 11, 12) comprise phasedetermining means (11, 12) for determining any phase shift introduced bythe transducer (7).
 3. The device according to claim 2, wherein thephase determining means comprise a combination unit (11) for combining afirst signal (V_(M), V₀′, V₀″) representative of the phase of thedriving signal's voltage and a second signal (V_(R)) representative ofthe driving signal's current so as to produce a phase difference signal(V_(D)), and a control unit (12) for producing a frequency controlsignal (V_(C)) on the basis of the phase difference signal (V_(D)). 4.The device according to claim 3, wherein the first signal is the drivingsignal (V_(M)).
 5. The device according to claim 3, wherein the firstsignal is an auxiliary oscillator signal (V₀′, V₀″).
 6. The deviceaccording to claim 5, further comprising a phase compensation unit (14)for introducing a compensatory phase shift in the auxiliary oscillatorsignal (V₀′) so as to produce a phase shifted auxiliary oscillatorsignal (V₀″).
 7. The device according to claim 3, further comprising aresistor (10) arranged in series with the transducer (7) for producingthe second signal (V_(R)) in response to the driving current (I_(L)). 8.The device according to claim 1, further comprising an accelerationdetector for detecting an acceleration of the transducer (7).
 9. Thedevice according to claim 1, further comprising a displacement detectorfor detecting a displacement of the transducer (7).
 10. The deviceaccording to claim 1, further comprising a generator (6) for generatingan oscillation signal (V₀) having a frequency substantially equal to theresonance frequency of the transducer, and a further combination unit(5) for combining the oscillation signal (V₀) with an amplitude controlsignal (V_(E)) so as to produce the amplitude controlled driving signal(V_(M)).
 11. The device according to claim 1, further comprising anamplifier (9) for amplifying the driving signal (V_(M)).
 12. The deviceaccording to claim 1, further comprising a low-pass filter (13) forfiltering the driving signal (V_(M)).
 13. A frequency adaptation device(1) for adapting a frequency range of an audio signal to a transducer(7), the device comprising a filter (2) for selecting a frequency range,a detector (3) for detecting signals in the selected frequency range,and a device for producing a driving signal (V_(M)) for a transducer (7)according to claim
 1. 14. A frequency adaptation device (1) for adaptinga frequency range of an audio signal to a transducer (7), the devicecomprising: detection means (3) for detecting first signal components ina first audio frequency range (I), generator means (6) for generatingsecond signal components in a second audio frequency range (II), andamplitude control means (5) for controlling the amplitude of the secondsignal components in response to the amplitude of the first signalcomponents, and control means (8; 10, 11, 12) for determining the secondaudio frequency range (II) on the basis of transducer properties,wherein the second audio frequency range (II) is substantially narrowerthan the first audio frequency range (1), and wherein the transducer (7)has a maximum sensitivity at the second audio frequency range (II). 15.The device according to claim 14, wherein the control means (8; 10, 11,12) are arranged for automatically controlling the second frequencyrange (II) on the basis of transducer properties.
 16. A method ofproducing a driving signal (V_(M)) for a transducer (7), the drivingsignal having a frequency substantially equal to a resonance frequencyof the transducer and an amplitude controlled by an external signal(V_(E)), the method comprising the step of automatically adjusting thefrequency of the driving signal to the resonance frequency of thetransducer.
 17. The method according to claim 16, further comprising thestep of determining any phase shift introduced by the transducer (7).18. The method according to claim 17, further comprising the steps ofcombining a first signal (V_(M), V₀′, V₀″) representative of the phaseof the driving signal's voltage and a second signal (V_(R))representative of the driving signal's current so as to produce a phasedifference signal (V_(D)), and producing a frequency control signal(V_(C)) on the basis of the phase difference signal (V_(D)).
 19. Themethod according to claim 18, wherein the first signal is the drivingsignal (V_(M)).
 20. The method according to claim 18, wherein the firstsignal is an auxiliary oscillator signal (V₀′, V₀″).
 21. The methodaccording to claim 20, further comprising the step of introducing acompensatory phase shift in the auxiliary oscillator signal (V₀′) so asto produce a phase shifted auxiliary oscillator signal (V₀″).
 22. Themethod according to claim 18, further comprising the step of producingthe second signal (V_(R)) in response to the driving current (I_(L)).23. The method according to claim 16, further comprising the step ofdetecting an acceleration of the transducer (7).
 24. The methodaccording to claim 16, further comprising the step of detecting adisplacement of the transducer (7).
 25. The method according to claim16, further comprising the steps of generating an oscillation signal(V₀) having a frequency substantially equal to the resonance frequencyof the transducer, and combining the oscillation signal (V₀) with anamplitude control signal (V_(E)) so as to produce the amplitudecontrolled driving signal (V_(M)).
 26. The method according to claim 16,further comprising the step of amplifying the driving signal (V_(M)).27. The method according to claim 16, further comprising the step offiltering the driving signal (V_(M)).
 28. A frequency adaptation methodfor adapting a frequency range of an audio signal to a transducer (7),the method comprising the steps of selecting a frequency range,detecting signals in the selected frequency range, and producing adriving signal (V_(M)) for a transducer (7) in accordance with claim 16.